[Coco] Cocovga 64 column not 80?

Brendan Donahe brendan at polylith.com
Wed May 1 20:58:05 EDT 2019


There were a number of reasons for this CoCoVGA FPGA design decision:

1.) I could conserve space in the FPGA by reusing the character ROMs Steve
Spiller and I re-implemented in the design, as opposed to creating another
new character set.
2.) By default in text mode or RG6/PMODE4, each CoCo video pixel consumes a
2x2 pixel of the VGA 640x480.  This means that resolution could be doubled
in each direction without exceeding the current VGA video format or doing
away with the border around the green field.
3.) Doubling the 32x16 = 512 byte text and semigraphics display
horizontally and vertically still enables nice power-of-2 (64x32 = 2048)
counters and sampling into the FPGA's video buffers.
4.) Sticking to a 8x12 character enables the software-uploadable custom
character sets to be used in both 32-column and 64-column modes.

I agree it was a bit of a compromise.  I would have preferred 80 columns,
but not at the expense to the design size and complexity.

Note that all of this logic has been re-implemented from the 6847.  In
other words, the VDG and CoCoVGA FPGA each have their own character


On Wed, May 1, 2019 at 7:02 PM Joel Rees <joel.rees at gmail.com> wrote:

> So, obnoxious question that it is, is it 64 column because it is using the
> internal character generator?
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